1. Field of the Invention
The present invention relates to a process for separating components of a gas by adsorption in an enclosure divided into equal tight separated compartments each provided with an adsorbent material chosen in function of the gas to be treated and each arranged for temporarily allowing the gas to be treated to be introduced and at least one chosen component of the components of this gas to be evacuated whilst the other component or components of this gas are adsorbed by said material, which process consists in introducing the gas to be treated into one of the compartments until a predetermined pressure is reached while in the next compartments, consecutively starting from the one which is the nearest to said compartment where the gas is pressurized, the following operations are performed: gas to be treated is introduced in at least one compartment and said chosen component is allowed to escape therefrom, the pressure in the next compartment is allowed to drop so as to obtain naturally a partial desorption of the non chosen component or components of the gas, optionally this same compartment is subsequently, also in view of the partial desorption, subjected to a partial vacuum and a purging fluid is injected in the last compartment to achieve the final desorption of the material.
2. Description of the Related Art
Separation of gasses is carried out by the following known techniques:
cryogenics PA1 selective adsorption of components of the gas on an adsorbent, through alternate cycles in two or several reactors each operating successively in adsorptive and desorptive mode, by temperature change effect (T.S.A. =Temperature Swing Adsorption) (see patent GB-871,242), or by pressure change (P.S.A. =Pressure Swing Adsorption). PA1 at least one additional compartment is provided between the compartment wherein the pressure is allowed to drop and the compartment wherein the purging fluid is injected, which additional compartment is subjected to a final desorption vacuum, if necessary stronger than the partial vacuum for the partial desorption, and the compartment wherein the purging fluid is injected is also subjected to substantially the same final desorption vacuum, and PA1 an additional compartment is provided between the compartment wherein the purging fluid is injected and which is subjected to a final desorption vacuum, and the compartment wherein the gas to be treated is introduced up to a predetermined pressure, in which additional compartment gas to be treated is let in naturally, by natural aspiration, compensating for the vacuum prevailing therein after the vacuum purging operation.
The cryogenic technique is applied for large quantities of gas to be treated. In this case, high tech centralised installations are involved due to the extremely low temperatures to be reached in order to obtain the liquefaction required for separating the components of the gas to be treated.
The cryogenic technique shows the important drawbacks of many exchangers and devices working at very low temperatures (-100.degree. to -190.degree. C.) and consuming a lot of energy. This process type is expensive as to capital outlay in that, in order to obtain a sufficient efficiency, large centralised units have to be constructed in order to profit from the "scale effect" and the pure gas has to be delivered to the users by means of an extensive network of pipings, or by transporting this gas in liquid form, which requires specific very expensive means.
The known process of the P.S.A. type (see "Separation and Purification Methods", 14(2), 1985; Marcel Dekker Inc. ; D. Tondeur, P. Wankat ; Gas Purification by P.S.A. ; pages 157, 160, 161) is based on the system shown in FIG. 1 and has been developed since 1970. This process and the means for carrying out this process are described hereinafter (see, for example, the UK patent 1,150,346 filed on Sep. 22, 1966).
This known P.S.A. process comprises (FIG. 1) two or more reactors A and B, filled with an adsorbent mass, a gas compressor 15 feeding reactor A with gas after being cooled in an exchanger 106 by opening a valve 101. The gas to be separated into its components (for example, air from which one wishes to extract oxygen) passes through reactor A, the adsorbent material being chosen for retaining preferentially one or more components of the air (for example, H.sub.2 O, N.sub.2). The gas becomes progressively impoverished in these components until they are almost entirely eliminated. The chosen component (oxygen, for example) leaves reactor A via the valve 103 towards the utilization 108. In the chosen example, it is therefore a gas released from its moisture and from nitrogen, which comes out of reactor A, i.e., a gas consisting of oxygen and argon. The non-chosen components are adsorbed and accumulate in the adsorbent mass which is quickly saturated. It is therefore necessary to regenerate the adsorbent. In order to prevent interruption of the production during the regeneration, a second or further reactors working alternately or substantially simultaneously are required. When adsorbing the component to be extracted (N.sub.2, for example) in reactor A, the same component adsorbed in the mass of reactor B is desorbed. To this end, reactor B, isolated from reactor A by closing the valves 111 and 113, is subjected to a pressure drop when opening a valve 112, the adsorbed gas being possibly extracted by a vacuum generated by the vacuum pump 6. The adsorbed gas is in this way progressively evacuated from the adsorbent mass by the pressure change effect. After a relatively short working period (1 to 3 minutes, for example), the adsorbent mass of reactor A is saturated while the adsorbent mass of reactor B is regenerated, and the gas circuits are reversed. Fresh gas is sent to reactor B and the desorption is performed in reactor A by reversing the valves 101,102,111, 112, 103, 113. Moreover, remaining gasses contained in the reactors are evacuated, by inverse sweeping of pure gas (oxygen, for example) by means of valves 104 and 114.
The known P.S.A. process described hereinabove, although it has been relatively recently developed, has the drawback of comprising a large number of valves working very frequently, i.e., 500,000 operations a year. This requires the use of high quality materials and an efficient maintenance service. The high number of reversing operations imposes a limited diameter of the valves and, consequently, a limited capacity of gas treatment. Additionally, the energy consumption, although less compared to the cryogenic technique, is still very important.
Other recent P.S.A. systems have been worked out. Patent application EP-A-0 512 534 filed on May 7, 1992 describes, for example, a P.S.A. system composed of a fixed or movable reactor, divided into compartments (2 to 8) and fed by one or two rotary horizontal flat valves rotating between fixed plates.
This last P.S.A. system has i.a. considerable drawbacks from the fact that it comprises one or two sliding surfaces (flat valves) disposed between fixed surfaces, which results in large leakages of gas and requires an expensive maintenance in order to prevent erosion and wear of these surfaces. Further, a considerable driving power is required for actuating these rotary devices. The energy consumption of this system is as high as the preceding P.S.A. processes.